Nanocomposites comprise very small particles typically having diameters less than 100 nm deposited on the surface of a support or within a host matrix. In recent years, nanocomposites have become a topic of great interest. When the particles are metal particles, the nanocomposites exhibit interesting electronic and nonlinear optical properties. When the particles are metallic with a high surface area, the nanocomposites exhibit high chemical reactivity as catalysts for a variety of chemical reactions. The average diameter of metallic nanoparticles usually can be controlled by varying annealing conditions or by metal loading. Generally, nanoparticle diameter can be increased by using high-temperature annealing. Various support matrices for nanocomposites may be chosen for various chemical uses. For example, the matrix may be porous and/or conductive for various catalytic applications, or for optical applications, a transparent matrix may be useful.
A fuel cell directly converts chemical energy of fuel and oxidant reactants into low-voltage direct current by means of electrode-catalyzed electrochemical reactions. However, unlike conventional batteries, fuel cells do not consume the materials comprising the electrodes. In a fuel cell system, the cell converts chemical energy into electricity without undergoing irreversible chemical change to the cell. Provided the catalyst remains active, a fuel cell can operate as long as it has a source of fuel and oxidant, and the reaction products are removed.
A fuel cell performs the same function as a galvanic cell or a discharging storage battery. However, in a fuel cell, the reactants are stored outside the reaction areas. The reactants are provided to the electrodes only when power generation is required. The oxidizing reactant in most fuel cells is atmospheric oxygen. When a fuel cell runs out of a reactant, generation of electricity ceases. Power generation resumes when the reactant is provided to the system.
A fuel cell comprises a fuel electrode (anode) and an oxidant electrode (cathode) separated by an ion-conducting electrolyte. The electrodes are conductively connected through a load by an external electronic circuit. The electric current is transported by the flow of electrons in the conductive part of the circuit. A simple hydrogen-oxygen fuel cell has the theoretical potential of generating direct current at a voltage up to 1.229 V at standard temperature and pressure.
The first successful fuel cells resulted from research conducted as a part of the United States aerospace program in the mid-1960's. By around 1970, several problems with fuel cells had become apparent. One problem was identifying a fuel that was efficient enough for practical use. Gaseous hydrogen was the only fuel that fell into this category. Hydrocarbons and alcohols had insufficient reactivity with even the most active catalysts, and other fuels, such as hydrazine, were not commercially feasible. Another problem with such fuel cells was that, for fuel cells constructed with aqueous electrolytes capable of handling reformed hydrocarbons, the only effective catalysts were expensive noble metal catalysts, such as platinum or palladium catalysts having relatively high loadings (about 25 mg/cm.sup.2 of noble metal). Because of the high cost, widespread commercial use of these noble metal catalysts was not practical. A third problem was the lifetime of the catalysts, which was initially only in the hundreds of hours, owing to catalyst degradation.
The energy crisis in 1974 and environmental concerns over fossil fuel related pollution awakened interest in fuel cell research. The products generated by fuel cells are less environmentally harmful than the products generated by combustion of fossil fuels. More recently, better understanding of electrocatalysis and improved techniques of catalyst preparation have made it possible to use electrodes having much lower platinum loadings without compromising fuel cell performance. As a result of this renewed research, catalyst loadings have been reduced to as low as 0.5 mg/cm.sup.2 of noble metal or less for hydrogen/oxygen fuel cells. Also, recovering and recycling fuel cell catalysts is now possible. Lifetimes of fuel cell electrode catalysts have also been greatly increased, from hundreds of hours to more than 50,000 hours.
Currently, many different aspects of fuel cell chemistry are being researched in an effort to enhance the efficiency of fuel cells including fuel cell design, the fuel type and oxidant type, as well as the type and activity of the catalyst. To design a fuel cell for practical, commercial use, the cells should be mechanically stable and not require cost-prohibitive fuels, design, or catalysts. Several types of fuel cells are generally accepted for practical application.
One type, hydrogen-oxygen fuel cells, are practical because the high electrochemical reactivity of hydrogen makes these cells the most efficient of all fuel cells yet designed. Hydrogen-oxygen fuel cells are also attractive because water is the only product, as indicated in the following reaction scheme: ##STR1##
While hydrogen has a high energy density and unlimited availability, so long as there is water available to decompose into hydrogen, it has the disadvantages of having a low mass density, creating storage difficulties for large quantities. It is also flammable and explosive.
A second type of fuel cell being studied for practical purposes is an indirect methanol-oxygen fuel cell. Indirect methanol-oxygen fuel cells rely upon steam reformation of methanol to produce a hydrogen-rich gas. The hydrogen reacts with oxygen in the cell to generate water: EQU 2H.sub.2 +O.sub.2.fwdarw.2 H.sub.2 O (IV)
While the indirect methanol-oxygen fuel cell may be an improvement over hydrogen-oxygen fuel cells with respect to less expensive and safer storage of the fuel (methanol), the portability of such fuel cells is limited by the size of the methanol reforming device required for the reaction and by the operating temperature of that device.
The third type of practical fuel cell is the direct methanol fuel cell (DMFC). The DMFC has the advantages of the indirect methanol fuel cell without sacrificing portability or safety. The DMFC electrochemically converts methanol into carbon dioxide and water without requiring an intermediate reformation of methanol into hydrogen gas. The overall reaction for this type of fuel cell is as follows: EQU CH.sub.3 OH+3/2O.sub.2.fwdarw.CO.sub.2 +2H.sub.2 O (V)
The products of this reaction are environmentally less harmful than products released by other commercial electricity generation methods. Unfortunately, the conversion rate of aqueous methanol is low using currently available catalysts. The rate of conversion increases in acidic media. However, this imposes an additional requirement: that the catalyst be chemically resistant to acid. Another shortcoming of existing DMFCs is that the platinum anode catalysts can become poisoned by a variety of methanol partial oxidation products. As such, there is a need in the art to develop a less poisoning-susceptible catalysts for such cells and to make the DMFCs more commercially feasible.
There is still a significant need in the art for improved DMFC catalysts, particularly anode catalysts. At the anode of such fuel cells, the following reaction takes place: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.- (VI)
The first anode catalysts used in DMFCs were platinum black. Platinum catalysts are fairly effective in DMFCs, but become poisoned by methanol partial oxidation products, thereby lowering the lifetime of the catalyst. Catalyst poisoning is believed to occur when partially oxidized products of methanol react and bond to the surface of the platinum catalyst, reducing the number of available catalytic sites. Continued poisoning eventually renders the catalyst useless.
A DMFC design includes two catalytic electrodes separated by a conductive ion-exchange membrane. The anode side of the cell is typically filled with a 0.5-2.0 molar aqueous methanol solution. On the cathode side, oxygen is bubbled through a low pH aqueous solution at a pressure of 1.0-5.0 atmospheres of oxygen. The reaction products, carbon dioxide and water, are continuously removed from the cell. The anode and cathode are connected by an electrical circuit. Methanol must be introduced into the anode in the vapor phase to avoid the problem of dehydration of the electrode membrane. The efficiency of DMFCs can be increased by raising the temperature of the fuel cell above 100.degree. C. which vaporizes the methanol and accelerates the rate of reaction (V) noted above. Use of metal catalysts supported on conductive supports of high surface area is preferred for practical applications.
Attempts to enhance reactivity at the anode include addition to the catalyst composition of another metal wherein a metal-oxygen bond energy similar to that of the platinum-carbon bond energy is postulated to reduce catalyst poisoning. Catalysts comprising the metal combinations platinum-ruthenium, platinum-rhenium, and platinum-tin were shown to exhibit enhanced catalytic activity relative to catalysts consisting of platinum alone.
There is still a need in the art for a commercially feasible method for making polymetallic nanocomposites in the form of well-dispersed, uniformly sized crystalline nanoparticles on a support and in which the metals are present in commercially desired stoichiometric ratios. This need in the art is the focus of intense research in DMFC catalysis. The purity of the nanoparticles, that is a high degree of dispersion with little or no variation of phases or impurities is important. The purity of the alloy is significant in that it enables successful methanol oxidation by avoiding poisoning and facilitating the methanol oxidation reaction.
Metal/carbon nanocomposites formed of metal nanoparticles supported on a carbon powder of high surface area are commonly used heterogeneous catalysts in both small- and large-scale chemical processes in various fields, particularly those involving hydrogenation/dehydrogenation reactions. Bird, In: Catalyst Supports and Supported Catalysts: Theoretical and Applied Concepts, Stiles, Ed., Buttersworth, Boston, pp. 107-137 (1987); Satterfield, In: Heterogeneous Catalysis in Industrial Practice, McGraw-Hill, New York (1991). Heterogeneous catalysts comprising bimetallic alloy nanoparticles as the active catalyst have been of great interest because of their unique chemical reactivity. Sinfelt, In: Bimetallic Catalysts: Discoveres, Concepts, and Applications, Wiley, New York (1983); Ponec, Adv. Catal. 32:149 (1983); Moser, Ed., In: Advanced Catalysts and Nanostructured Materials, Academic Press, San Diego (1996).
Such carbon-supported nanoparticles, particularly platinum and platinum-rich alloy nanoparticles, such as platinum-ruthenium, are useful fuel cell catalysts. Catalysts that have been studied include PtRu, Pt.sub.3 Ru, and PtPb. Gasteiger et al., J. Electrochem. Soc. 141:1796 (1994); Hamnett et al., J. Electrochimica Acta 33:1613 (1988); Watanabe et al., J. Electroanal. Chem. 229:396 (1987). These catalysts have also been investigated for use as anode catalysts in fuel cells. For a fuel cell to work efficiently, a good cathode catalyst is also required. Pt--Ru nanoparticles supported on an electrically conductive carbon support of high surface area (such as VULCAN.TM. Carbon XC-72R, Cabot Corporation) are now recognized as highly reactive anode catalysts in DMFCs. Hamnett, Catal. Today 38:445 (1997); Hogarth et al., Platinum Metals Rev. 40:150 (1996); Chandler et al., Platinum Metals Rev. 41:54 (1997); Ralph, Platinum Metals Rev. 41:102 (1997); Hamnett et al., Chem. Ind. 480 (1992); Ren et al., Electrochem. Soc. 143:L12 (1996); Hogarth et al., Proc. First Intern. Symp. New Mater. Fuel Cells 310 (1995); Lin et al., J. Electrochem. Soc. 144:1917 (1997); Gasteiger et al., Electrochem. Soc. 141:1795 (1994); Surampudi et al., J. Power Sources 47:377 (1994); Wang et al., Electrochim. Acta 41:2587 (1996); Pathanjali et al., Bull. Electrochem. 12:193 (1996).
Binary Pt--Ru/carbon catalysts wherein the metal alloy comprises at least 50 atomic percent platinum have been found to exhibit superior activity for fuel cells relative to that of Ru-rich catalysts. Gasteiger et al., J. Electrochem. Soc. 141:1795 (1994). Pt.sub.1 Ru.sub.1 /Vulcan carbon powder nanocomposites are now commercially available, e.g. from ElectroChem, Inc., Woburn, Mass.; E-TEK, Inc., Natick, Mass.; Electrosynthesis Co., Inc., Lancaster, N.Y.; and, Johnson Matthey Plc., Hertfordshire, England.
Carbon-supported metal or metal alloy nanocomposites containing Pt (or Pd) are commonly prepared using various impregnation/reduction or chemical vapor deposition methods. A typical procedure involves impregnation of powdered carbon by solutions of Pd or Pt salts followed by drying and subsequent chemical reduction of the metal ions to the metal. Bird, In: Catalyst Supports and Supported Catalysts: Theoretical and Applied Concepts, Stiles, Ed., Buttersworth, Boston, pp. 107-137 (1987); Satterfield, In: Heterogeneous Catalysis in Industrial Practice, McGraw-Hill, New York (1991). Mixtures of hydrogen and nitrogen gases, such as 10 percent by volume hydrogen and 90 percent by volume nitrogen (known as "getter gas") are common reducing agents. Pt--Ru/Vulcan carbon composites are usually synthesized by either co-deposition of Pt and Ru from aqueous solutions of platinum and ruthenium salts upon chemical reduction or by co-deposition of Pt and Ru from aqueous solutions of the sulfito complexes, N.sub.6 [Pt(SO.sub.3).sub.4 ] and Na.sub.4 [Ru(SO.sub.3).sub.3 ], following the Watanabe procedure. Watanabe et al., Electroanal. Chem. Interfacial Electrochem, 229:395 (1987).
Metal alloy/carbon nanocomposites have also been prepared using cluster complexes as molecular precursors to attempt to better control the relative metal stoichiometry of the metal alloy particles. Ichikawa, M. Adv. Catal. 38:283 (1992). Fe--Ru and Fe--Mn/carbon composites have been prepared from mixed-metal carbonyl cluster complexes as precursors. Kaminsky et al., J. Catal. 91:338 (1985); Venter et al., J. Catal. 103:450 (1987). In cluster-type polymetallic precursors, at least one atom of one of the metals is directly bonded to an atom of the second metal in the cluster. In addition, Pt--Ru composites with a PtRu.sub.5 stoichiometry on carbon have been formed from a PtRu.sub.5 C(CO).sub.16 cluster precursor. Nashner et al., J.Am.Chem.Soc. 119, 7760-7771 (1997). However, there is still a need for nanocomposites and precursors for making such composites which are commercially economical and which provide desirable and controlled mixed metal nanocluster stoichiometries of at least a 1:1 ratio and preferably with at least 50% platinum.
Significant needs remain for improved catalysts suitable for uses such as anode catalysis in DMFCs and for methods of making such improved catalysts. The present invention satisfies these needs.